The radio-frequency transceiver is ubiquitous and far ranging in the applications to which it is applied. Shortwave radios allow licensed operators to communicate across oceans and continents. Citizen band (CB) radios are generally less regulated but more common. Pilots rely on radio-frequency transceivers to communicate with air traffic controllers who coordinate the access to limited airspace. Marine vessels use the transceivers to request and receive weather information, or in the event of an emergency, request assistance from the coast guard or nearby ships. And, this merely represents a short list of the applications for these devices.
Common to every application of the radio frequency transceiver is the need for radio checks. Most radio-frequency transceivers do not provide any independent confirmation of the establishment of a radio link between two communicating parties. More simply, when operators transmit a request for information, for example, the only confirmation that this request is heard and intelligible is the response from another party. As a result, if the party receives no response from a given transmission, either of two events could have occurred: 1) no one in fact responded to the request, or 2) the transmitter and/or receiver portions of the transceiver are not operating properly. As a result, in order to exclude the second possibility, the operator periodically requests radio checks to ensure that the transceiver is fully operational. According to this process, a given operator will broadcast on the frequency and request that someone respond. Another party upon hearing this request will transmit back that the operator's request was received and give some indication of the strength of the received signal. Thus, the operator is assured that the transceiver is operating properly: 1) the transmitter portion effectively transmitted a signal since it was received by the other party, and 2) the receiver portion operates properly since the other party's transmission was received.